1. Field of the Invention
The present invention relates generally to audio/video (AV) systems, and more specifically, to a method and system for generating an audio clock source from a detected video clock rate embedded in a video signal that is sampled asynchronously by a video receiver.
2. Background of the Invention
Digital audio and video systems are prevalent in the areas of home entertainment, professional production of multimedia and computer reproduction and generation of multimedia sources. In particular, systems that record a video stream are frequently coupled to source devices such as cameras and also produced from AV playback devices such as DVD and CD players.
It is necessary to receive, resynchronize and sometimes resample a video signal for recording or processing purposes, whether by a general-purpose computing device, a dedicated video processing device, or a dedicated video recording device. The source video rate may not match the recording or processing video rate, and even if the rates did match, it would be desirable to asynchronously sample the video signal for in order to stabilize the video data to a precision reference clock.
In general, less noise and jitter is generally provided by asynchronously sampling a signal, rather than synchronously sampling a signal, as a very stable local reference clock may be employed. The local reference clock can be designed to minimize environmentally-introduced noise, be ideally terminated to the sample clock input and otherwise be optimized for the video sampling circuit. However, asynchronous sampling of signals with accurate phase preservation requires a high oversampling factor, which is typically impractical for video sources. In particular, the recovery of the synchronization elements in a video signal (such as the luminance and chrominance carriers that require phase accuracy on the order of two degrees or less) would require a sampling clock on the order of 1 GHz in order to completely preserve the video signal.
Traditionally, video clock sources are sampled synchronously with a source-locked video clock. The source-locked video clock used to sample the video signal is generated by a phase-lock loop (PLL) that locks to the incoming line rate, frame rate, or combination of both. While source-locking provides a video clock that is free of drift or “slipping”—at least for relatively clean video signals, video degradation is present in the sampled video due to jitter and other noise that is introduced or cannot be removed by the PLL. In particular, most composite video signals have events such as vertical blanking interval (VBI) that require special circuits in the PLL to avoid disrupting the lock signal and the frequency of the video clock signal of the source may vary from source to source over a fairly wide margin. PLLs that handle such variable signals and disruptive events require a reasonably wide loop bandwidth for capture and lock retention.
To solve this problem with asynchronous sampling while providing its benefits, the above-incorporated patent application “METHOD AND SYSTEM FOR SYNCHRONIZING VIDEO INFORMATION DERIVED FROM AN ASYNCHRONOUSLY SAMPLED VIDEO SIGNAL” provides an asynchronously sampled video reception system and method that do not require a line-locked clock. The disclosed system provides a mechanism for fully-digital recovery of the embedded source clock information, allowing reintroduction of the original synchronization to high-quality asynchronously sampled video.
The above-incorporated patent application further provides the benefit of independent conversion of the incoming video clock rate to another clock rate unrelated to the video source clock.
However, audio information provided in conjunction with a video signal is sampled by an audio subsystem having its own reference clock. Without a means for synchronizing the audio sampling clock (reference audio clock) to the video, drift occurs between audio and video events, eventually leading to perceptible differences that ruin the playback value of recorded multimedia streams.
Further, in digital video-audio processing and recording systems, the desired audio output clock rate may be a different rate than a rate associated with the source video clock or the rate to which the asynchronously sampled video is being converted in the system disclosed in the above-incorporated patent application. Common rates for processing and recording systems are usually based on the CD-Audio or DAT audio standards, with clock rates of 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz, 192 kHz and the like. The sampled audio is either left in raw mode (PCM) or converted via a codec to a compressed stream at one of the above rates or another dedicated rate.
Therefore, it would be desirable to provide a method and system for generating an arbitrary rate reference audio clock from a video source stream that has been asynchronously sampled.